Cutting insert and method for production thereof

ABSTRACT

Cutting insert made of hard metal, cermet or ceramic substrate body with multi-layer coating applied thereto by CVD methods. The coating has a total thickness of 5 to 40 μm and, starting from the substrate surface, has one or more hard material layers, an alpha aluminum oxide (α-Al 2 O 3 ) layer of a layer thickness of 1 to 20 μm and optionally at least portion-wise over the α-Al 2 O 3  layer one or more further hard material layers as decorative or wear recognition layers. The α-Al 2 O 3  layer has a crystallographic preferential orientation characterized by a texture coefficient TC (0 0 12)≧5 for the (0 0 12) growth direction. 
     The α-Al 2 O 3  layer has an inherent stress in the region of 0 to +300 MPas, and the substrate within a region of 0 to 10 μm from the substrate surface has an inherent stress minimum in the region of −2000 to −400 MPas.

SUBJECT OF THE INVENTION

The invention concerns a coated cutting insert made of a hard metal,cermet or ceramic substrate body and a multi-layer coating which isapplied thereto by means of CVD methods and which starting from thesubstrate surface has one or more hard material layers, over the hardmaterial layers an alpha aluminium oxide (α-Al₂O₃) layer and optionallyat least portion-wise over the α-Al₂O₃ layer one or more further hardmaterial layers as decorative or wear recognition layers.

BACKGROUND OF THE INVENTION

Cutting inserts for material working, in particular for cutting metalworking, comprise a hard metal, cermet or ceramic substrate body whichin most cases is provided with a single-layer or multi-layer surfacecoating to improve the cutting and/or wear properties. The surfacecoatings comprise mutually superposed hard material layers of carbides,nitrides, oxides, carbonitrides, oxynitrides, oxycarbides,oxycarbonitrides, borides, boronitrides, borocarbides,borocarbonitrides, borooxynitrides, borooxocarbides orborooxocarbonitrides of the elements of groups IVa to VIIa of theperiodic system and/or aluminium, mixed metal phases and phase mixturesof the afore-mentioned compounds. Examples of the afore-mentionedcompounds are TiN, TiC, TiCN and Al₂O₃. An example of a mixed metalphase in which in a crystal a metal is partially replaced by another isTiAlN. Coatings of the afore-mentioned kind are applied by CVD methods(chemical vapour phase deposition), PCVD methods (plasma-supported CVDmethods) or by PVD methods (physical vapour phase deposition).

Inherent stresses obtain in almost every material as a consequence ofmechanical, thermal and/or chemical treatment. In the production ofcutting inserts by coating a substrate body by means CVD methods,inherent stresses result for example between the coating and thesubstrate and between the individual layers of the coating from thedifferent coefficients of thermal expansion of the materials. Theinherent stresses can be tension or compression inherent stresses. Whena coating is applied by means of PVD methods additional stresses areintroduced into the coating by ion bombardment when using that method.In coatings applied by means of PVD methods compression inherentstresses generally prevail whereas CVD methods usually produce tensioninherent stresses in the coating.

The effect of the inherent stresses in the coating and in the substratebody can be without a considerable influence on the properties of thecutting insert, but they can also have considerable advantageous ordisadvantageous effects on the wear resistance of the cutting insert.Tension inherent stresses which exceed the tensile strength of therespective material cause fractures and cracks in the coatingperpendicularly to the direction of the tension inherent stress. Ingeneral a certain amount of compression inherent stress in the coatingis desired as surface cracks are prevented or closed thereby and thefatigue properties of the coating and thus the cutting insert areimproved. Excessively high compression inherent stresses however canlead to adhesion problems and spalling of the coating.

There are 3 kinds of inherent stresses: macrostresses which are almosthomogenously distributed over macroscopic regions of the material,microstresses which are homogenous in microscopic regions like forexample a grain, and non-homogenous microstresses which are alsonon-homogenous on a microscopic plane. From a practical point of viewand for the mechanical properties of a cutting insert macrostresses areof particular significance.

Inherent stresses are usually specified using the unit Megapascal (MPa),wherein tension inherent stresses have a positive sign (+) andcompression inherent stresses have a negative sign (−).

It is known that hard metal cutting tools which are coated with hardmaterial layers like for example TiN, TiC, TiCN, TiAlN, Al₂O₃ orcombinations thereof can have excellent wear resistance but they canrather fail in a situation involving thermomechanical alternatingloading in interrupted cutting operations as for example in crankshaftmilling, by virtue of a loss in toughness in relation to uncoatedcutting tools or those which are coated by means of PVD methods. Asimilar consideration applies to turning working in an interruptedcutting mode or under disadvantageous cutting conditions (for examplevibrations caused by the machine or the workpiece clamping). For suchapplications under disadvantageous conditions, hitherto CVD coatingswith a limited layer thickness (rarely more than 10 μm) are used as theembrittlement of the cutting material, caused inter alia by tensilestresses, increases with the thickness of the CVD coating. Highlywear-resistant kinds of cutting materials in contrast frequently involvelayer thicknesses of 20 μm or more, but they can only be used in acontinuous cutting mode under advantageous conditions. In the case ofcutting inserts for turning working of steel or cast iron therefore bothhigh wear resistance and also high toughness are desired, these beingtwo properties which frequently cannot be achieved at the same time.

DE-A-197 19 195 describes a cutting insert have a multi-layer coatingwhich is deposited in a continuous CVD method at temperatures between900° C. and 1100° C. The change in material in the multi-layer coatingfrom one layer to the next occurs due to a change in the gas compositionin the CVD method. The outermost layer (cover layer) comprises asingle-phase or multi-phase layer of carbides, nitrides or carbonitridesof Zr or Hf, in which internal compression inherent stresses prevail.The subjacent layers comprise TiN, TiC or TiCN and without exceptionhave internal tension inherent stresses. The compression inherent stressmeasured in the outer layer is between −500 and −2500 MPas. That isintended to improve fracture toughness.

To increase the compression inherent stresses in the coating on thesubstrate body of cutting inserts or other tools it is known for them tobe subjected to a mechanical surface treatment. Known mechanical surfacemethods are brushing and jet blasting treatment. Jet blasting treatmentinvolves directing a fine-grain jet blasting agent of grain sizes of upto about 600 μm by means of compressed air under increased pressure onto the surface of the coating. Such a surface treatment can reducetension inherent stresses or compression inherent stresses in theoutermost layer and also in the subjacent layers. In regard to jetblasting treatment a distinction is drawn between dry jet blastingtreatment in which the fine-grain jet blasting agent is used in the drycondition and wet jet blasting treatment in which the granular jetblasting agent is suspended in a liquid.

It was found that the choice of the jet blasting agent has aconsiderable influence on the changes in the inherent stresses in thecoating and in the substrate of the cutting insert, in particular thehardness of the jet blasting agent in relation to the hardness andthickness of the coating. It was possible to show that, when using a jetblasting agent whose hardness is greater than that of the outermostlayer of the coating, the wear mechanism in the jet blasting procedureis abrasion and high compression stresses occur only at the near surfaceregions of the layer to about 1 μm depth of penetration, and they veryquickly relax again. In deeper layers or in the substrate there issubstantially no reduction in the tension stresses or increase in thecompression stresses. The inherent stress prevailing in the substrateafter the coating process remains unchanged. It is not possible toachieve an increase in the toughness of the tool.

If the hardness of the jet blasting agent is equal to the hardness ofthe outermost layer of the coating then the wear mechanism in the jetblasting operation is surface spalling and there are high compressionstresses which can act into deeper coating layers and in dependence onthe layer thickness also into the substrate. With thick layers (>>10 μm)with wet jet blasting the stress in the substrate can be only littlealtered and tensile strength can be increased. If nonetheless there is awish to increase the compression stress in the substrate even with thicklayers, it is necessary to use very long dry jet blasting operations,which leads to an increase in lattice dislocations and can causeadhesion problems with the coating.

If the hardness of the jet blasting agent is less than that of theoutermost layer of the coating surface bombardment (shot peening) isalso substantially assumed as the wear mechanism of that outermostlayer. The wear rate at the outermost coating is lower so that longerjet blasting times are possible without any layer removal worthmentioning. A further advantage is that in that respect no or onlyslight degrees of dislocation are produced in the uppermost layers ofthe coating. Depending on the respective choice of the method parameters(inter alia jet blasting agent, pressure, duration and angle) and layerthickness inherent stress changes can be achieved in different depths ofthe composite consisting of the hard metal and the coating. In otherwords, as a result of the jet blasting treatment, compression stressescan occur in different layers of the coating and also in the substrate.

DE-A-101 23 554 describes a jet blasting method using a granular jetblasting agent of a maximum diameter of 150μ. As a result, a reductionin tension inherent stresses or an increase in compression stresses isachieved in the outermost layer and the subjacent layers, preferably inthe region near the surface of the substrate. Preferably compressionstresses of some GPa are achieved in the uppermost layers.

Cutting inserts with an outer wear protection layer of alpha or gammaaluminium oxide for metal working have been in use for many years andare described in detail in the literature. It has been found that alphaaluminium oxide coatings with given preferential directions of crystalgrowth in deposition in the PVD or CVD methods can have particularadvantages, in particular an improved wear characteristic, in whichrespect for different applications of the cutting insert differentpreferential orientations of the aluminium oxide layer can also beparticularly advantageous. The preferential orientation of crystalgrowth is generally specified in relation to the planes defined by wayof the Miller indices, for example the (001) plane, of the crystallattice and are referred to as texture or fibre texture and are definedby way of a so-called texture coefficient (TC). For example cuttinginserts with a wear layer of alpha aluminium oxide with (001) texturehave advantages over other preferential orientations in steel machiningin respect of relief face wear and crater wear as well as plasticdeformation.

US-A-2007/0104945 describes cutting tools with α-Al₂O₃ wear layers with(001) texture and a columnar microstructure. That preferentialorientation is revealed by high intensities of the (006) peak in theX-ray diffraction spectrum (XRD diffractogram) and is achieved by bothnucleation and also growth of the α-Al₂O₃ layer being performed in theCVD method under given conditions. Nucleation is effected at ≦1000° C.on a TiAlCNO bonding layer by a multi-stage method in which thesubstrates are successively exposed to defined gas concentrations ofTiCl₄ and AlCl₃, flushing steps in N₂ and defined H₂O concentrations.Nucleation of α-Al₂O₃ is then continued by growth without catalyticadditives and finally at 950 to 1000° C. layer growth to the desiredlayer thickness takes place under a defined concentration ratio ofCO/CO₂ and in the presence of typical catalysts like H₂S, SO₂ or SF₆, inconcentrations≦1% by volume.

EP 1 953 258 also describes cutting tools with α-Al₂O₃ wear layers witha (001) texture on hard metal substrates with an edge zone enriched withCo binder. The preferential orientation of the α-Al₂O₃ wear layer isachieved by nucleation similarly to US-A-2007/0104945, but it will benoted that as a departure therefrom upon further growth of the layer theCO/CO₂ ratio gradually increases.

EP-A-2 014 789 also describes cutting tools with α-Al₂O₃ wear layerswith a (001) texture on hard metal substrates with an edge zone enrichedwith Co binder, which are said to be suitable in particular for cuttingmachining of steel at high cutting speeds, in particular for steelturning.

OBJECT OF THE INVENTION

The object of the present invention is to provide cutting inserts forcutting metal working, in particular turning working of steel or castmaterials, which have a wear resistance that is improved in comparisonwith the state of the art, in particular increased resistance at thesame time to wear forms which occur under a continuous loading, likerelief face wear, crater wear and plastic deformation, and also inrelation to wear forms which occur with a thermomechanical alternatingloading like break-offs, fractures and comb cracks, and which thusafford a broader area of application than known cutting inserts.

DESCRIPTION OF THE DRAWING FIGURES

-   -   FIG. 1 shows the tool according to the state of the art (cutting        insert 11 of Table 5) after the machining of 54 components.    -   FIG. 2 shows the cutting insert according to the invention        (cutting insert 14 of Table 5) after the machining of 80        components.

DESCRIPTION OF THE INVENTION

That object is attained by a cutting insert made of a hard metal, cermetor ceramic substrate body and a multi-layer coating which is appliedthereto by means of CVD methods of a total thickness of 5 to 40 μm andwhich starting from the substrate surface has one or more hard materiallayers, over the hard material layers an alpha aluminium oxide (α-Al₂O₃)layer of a layer thickness of 3 to 20 μm and optionally at leastportion-wise over the α-Al₂O₃ layer one or more further hard materiallayers as decorative or wear recognition layers, wherein

the α-Al₂O₃ layer has a crystallographic preferential orientation,characterised by a texture coefficient TC (0 0 12)≧5 for the (0 0 12)growth direction with

${{{TC}\;\begin{pmatrix}0 & 0 & 12\end{pmatrix}} = {\frac{I\begin{pmatrix}0 & 0 & 12\end{pmatrix}}{I_{0}\begin{pmatrix}0 & 0 & 12\end{pmatrix}}\left\lbrack {\frac{1}{n}{\overset{n}{\sum\limits_{n - 1}}\frac{I({hkl})}{I_{0}({hkl})}}} \right\rbrack}^{- 1}},$wherein

I(hkl) are the intensities of the diffraction reflections measured byX-ray diffraction,

I₀(hkl) are the standard intensities of the diffraction reflections inaccordance with pdf card 42-1468,

n is the number of reflections used for the calculation, and

the following reflections are used for the calculation of TC(0 0 12):

-   -   (0 1 2), (1 0 4), (1 1 0), (1 1 3), (1 1 6), (3 0 0) and (0 0        12),        the α-Al₂O₃ layer has an inherent stress in the region of 0 to        +300 MPas, and        the substrate within a region of 0 to 10 μm from the substrate        surface has an inherent stress minimum in the region of −2000 to        −400 MPas.

It was surprisingly found that, in a cutting insert with a coating ofthe kind described herein, in cutting metal working, in particular inthe turning working of steel or cast materials, it is possible toachieve a wear resistance which is improved over known cutting insertsand a broader area of application if the hard α-Al₂O₃ layer serving asthe wear layer has a crystallographic preferential orientation with atexture coefficient TC (0 0 12)≧5 and at the same time a low tensioninherent stress in the range of 0 to +300 MPas or even a compressioninherent stress and at the same time the substrate in a zone whichextends from the substrate surface to a depth of penetration of 10 μmand which is referred to as the “near interface substrate zone” of thesubstrate body has a compression inherent stress in the region of −2000to −400 MPas.

The combination according to the invention of the crystallographicpreferential orientation of the α-Al₂O₃ layer and the defined parametersof the inherent stresses of the α-Al₂O₃ layer and the substrate body inthe near interface substrate zone affords cutting inserts which aredistinguished in that they both have increased resistance to forms ofwear which occur in a continuous loading like relief face wear, craterwear and plastic deformation, and also forms of wear which occur inthermomechanical alternating loading like break-offs, fracture and combcracks. In comparison known cutting inserts are generally designed for agiven kind of loading and are optimised for same and thereforefrequently have a limited and highly specific area of application. Thecutting insert according to the invention in contrast, by virtue of itsincreased resistance to various forms of wear, namely those which occurpredominantly in a continuous loading situation and those which occurpredominantly in a thermomechanical alternating loading situation, has abroader area of application than known cutting inserts.

In a preferred embodiment of the cutting insert according to theinvention the production of the cutting insert includes the substratebeing subjected to a dry or wet jet blasting treatment, preferably a dryjet blasting treatment, using a granular jet blasting agent, afterapplication of the multi-layer coating, wherein the jet blasting agentpreferably has a lower level of hardness than corundum (α-Al₂O₃).

The inherent stresses according to the invention in the Al₂O₃ layer andin the substrate body of the cutting insert can advantageously beachieved by the cutting insert being subjected to a dry or wet jetblasting treatment using a granular jet blasting agent, afterapplication of the multi-layer coating to the substrate. In that casethe jet blasting agent should be of lesser hardness than corundum(α-Al₂O₃), in particular if the multi-layer coating is of greatthickness. For example particles of steel, glass or zirconium dioxide(ZrO₂) are suitable as the jet blasting agent. The jet blastingtreatment is desirably carried out at a jet blasting agent pressure of 1bar to 10 bars. The use of a jet blasting agent which is of lesserhardness than corundum in the above-mentioned pressure range has theadvantage that in that case no or only slight degrees of dislocation areincorporated into the uppermost layers of the coating. The α-Al₂O₃ layerand the subjacent layers of the coating exhibit only little change intheir inherent stresses.

The dry jet blasting treatment is particularly preferred as it ensures amore uniform application of the jet blasting pressure to the coating andthe substrate body over the entire surface, than the wet jet blastingtreatment. In the wet jet blasting treatment the formation of a film ofliquid on the jet-blasted surface considerably damps down theimplementation of inherent stresses in relation to dry jet blastingtreatment, with comparable jet blasting pressure conditions. That givesrise to the danger that the application of the jet blasting pressure atthe edges of tool, that is to say also at the important cutting edges,is substantially higher than to the smooth surfaces, which can have theresult that the edges are damaged under the jet blasting pressure beforethere is at all a substantial or at least adequate application to thesurfaces of the tool, that are essential for the cutting operations, inparticular the rake face. Higher pressures are also possible, over along period of time, by means of dry jet blasting treatment, without thetool being damaged thereby.

The duration of the jet blasting treatment, that is required forproducing or setting the inherent stresses according to invention in theα-Al₂O₃ layer and the substrate body, and the required jet blastingpressure, are parameters which the man skilled in the art can determinewithin the limits defined herein, by simple experiments on non-blastedcutting inserts. Comprehensive information is not possible here as theinherent stresses which occur depend not only on the duration of the jetblasting treatment and jet blasting pressure, but also on the structureand thickness of the overall coating and also the composition andstructure of the substrate. It will be noted in that respect that, incomparison with the jet blasting duration, the jet blasting pressure hasthe substantially greater influence on the change in the inherentstresses in the coating and the substrate body. The duration of the jetblasting treatment may not be too short so that the desired changes inthe inherent stresses penetrate into the substrate body and the inherentstress values according to invention can be set. The optimum duration ofthe jet blasting treatment also depends on the installation used forsame, the spacing, the nature and the orientation of the jet blastingnozzles and the movement of the nozzles over the blasted tool. Jetblasting treatment durations suitable for production of the cuttinginsert according to the invention are in the region of 10 to 600seconds, but they can also be in the region of 15 to 60 seconds.Particularly if one or more layers over the α-Al₂O₃ layer are to befirstly removed by the jet blasting treatment a longer jet blastingtreatment duration is desirable or required. Suitable jet blasting agentpressures are in the region of 1 to 10 bars, preferably 2 bars to 8bars, particularly preferably 3 bars to 5 bars. The invention however isnot restricted to the above-mentioned jet blasting treatment durationsand jet blasting agent pressures.

The jet blasting agent can be for example steel, glass or ZrO₂. Theinherent stress conditions according to the invention can be set withany of the stated or other suitable jet blasting agents. With knowledgeof the invention, the man skilled in the art can select a medium whichis desirable from method, technical installation or tribological pointsof view, and can arrive at suitable jet blasting parameters by simpletests. Preferably the jet blasting agent comprises spherical particles.The mean grain size of the jet blasting agent is desirably in the regionof 20 to 450 μm, preferably 40 to 200 μm, particularly preferably 50 to100 μm, but it does not have any substantial influence on the productionof compression inherent stresses in the substrate body. However the meangrain size of the jet blasting agent influences the surface roughness ofthe outermost layer of the coating. A small mean grain size (finegrains) produces a smooth surface in the blasting operation whereas ahigh mean grain size gives a rough surface. The production of a smoothsurface and thus the use of a jet blasting agent with a low mean grainsize is thus preferred for the tools according to the invention. TheVickers hardnesses of the above-mentioned jet blasting agents areapproximately in the region of 500 to 1500. According to the inventionAl₂O₃ (corundum) is generally not suitable as the jet blasting agent.

The jet blasting angle, that is to say the angle between the treatmentbeam and the surface of the tool, also has a substantial influence onthe introduction of compression inherent stresses. The maximumintroduction of compression inherent stresses occurs with a jet angle of90°. Lesser jet angles, that is to say inclined incidence of the jetblasting agent, result in more severe abrasion of the surface and alesser degree of compression inherent stress introduction. The mostsevere abrasion action is achieved with jet angles of about 15° to 40°.With smaller jet angles it may be necessary to adopt a higher jetblasting pressure and/or a longer jet blasting time in order to achievethe introduction of compression inherent stresses, that corresponds tothe introduction of such stresses with a jet blasting angle of 90°, withwhich the examples described herein were also performed. With knowledgeof the invention however the man skilled in the art can easily ascertainthose parameters which are to be applied when using smaller jet angles.

The term “surface-near region” of the substrate body denotes a regionfrom the outermost surface of the substrate body to a depth ofpenetration of a maximum of 1 to 2 μm in the direction of the interiorof the substrate body. Non-destructive and phase-selective analysis ofinherent stresses is effected by means of X-ray diffraction methods.Angle-dispersive measurement in accordance with the sin² ψ method, thatis widely used, delivers a mean value for the inherent stress componentin one plane and in WC substrates allows inherent stress measurementsonly to very small depths of penetration of a maximum of 1 to 2 μm fromthe surface, that is to say only in the “surface-near region” of thesubstrate body.

The term “near interface substrate zone” of the substrate body denotes aregion from the outermost surface of the substrate body to a depth ofpenetration of about 10 μm in the direction of the interior of thesubstrate body. Analyses of the inherent stress configuration in the“near interface substrate zone” were not possible with the previouslyapplied method of angle-dispersive measurement with conventionallaboratory sources. On the one hand, as mentioned above, the depth ofpenetration of the angle-dispersive measurement is limited to an onlyvery short distance from the outermost surface of the substrate body. Inaddition angle-dispersive measurement in accordance with the sin² ψmethod only supplies a mean value in one plane, for which reason thismethod cannot be used to measure stepwise changes or gradient variationsin the inherent stresses within short distances, with that method. Forthe analysis of inherent stresses in the “near interface substrate zone”of the substrate body to a depth of penetration of about 10 μmtherefore, an energy-dispersive measurement procedure was used for thecutting inserts of the general kind set forth, such energy-dispersivemeasurement allowing the analysis of inherent stress variations to adepth of penetration of about 10 μm while detecting the variation in theinherent stresses within that region.

The coating on the cutting insert according to the invention comprises asuccession of different individual layers. Because of their differingcompositions, production conditions and positions within the coating,those different layers generally also already involve different inherentstresses, that is to say tension or compression stresses of differingmagnitudes, prior to the jet blasting treatment. Due to the jet blastingtreatment the inherent stresses in the individual layers in turn changeby virtue of their differing compositions, production conditions andpositions within the coating, to differing degrees. A correspondingconsideration also applies to the substrate where the inherent stressesand changes therein at different depths from the surface can also be ofdiffering magnitudes. According to the invention measurement of theinherent stresses is limited to a region from the substrate surface to adepth of penetration of 10 μm. In WC substrates measurement of theinherent stresses in much greater depths is technically impossible.

In a preferred embodiment of the invention the hard material layersarranged over the substrate surface and under the α-Al₂O₃ layer and thehard material layers arranged at least portion-wise optionally over theα-Al₂O₃ layer comprise carbides, nitrides, oxides, carbonitrides,oxynitrides, oxycarbides, oxycarbonitrides, borides, boronitrides,borocarbides, borocarbonitrides, borooxynitrides, borooxocarbides orborooxocarbonitrides of the elements of groups IVa to VIIa of theperiodic system and/or aluminium and/or mixed metal phases and/or phasemixtures of the afore-mentioned compounds.

In a further preferred embodiment of the invention the hard materiallayers arranged over the substrate surface and under the α-Al₂O₃ layercomprise TiN, TiCN and/or TiAlCNO, wherein the hard material layersrespectively involve layer thicknesses in the region of 0.1 μm to 15 μm.

In particular the layer of TiAlCNO is suitable as a binding layerdirectly under the α-Al₂O₃ layer. If a hard material layer of TiAlCNO isarranged directly under the α-Al₂O₃ layer it is preferably of a layerthickness in the range of 0.1 μm to 1 μm. The TiAlCNO layer improves theadhesion of the α-Al₂O₃ layer and promotes growth of the aluminium oxidein the alpha modification and with the preferential orientationaccording to the invention. By virtue of its composition andmicrostructure it affords excellent binding to the TiCN layer. Goodbinding of the layers to each other is important to be able to applyhigh pressures in the jet blasting treatment without the layers spallingoff.

Hard material layers of TiN or TiCN, if there are one or more thereof,preferably involve layer thicknesses in the region of 2 μm to 15 μm,particularly preferably in the region of 3 μm to 10 μm.

Preferably, arranged under the binding layer of TiAlCNO and under theα-Al₂O₃ layer is a TiCN layer which is desirably of the above-mentionedlayer thickness in the region of 2 μm to 15 μm, preferably in the regionof 3 μm to 10 μm. The TiCN layer is preferably applied using a hightemperature CVD method (HT-CVD) or a medium temperature CVD method(MT-CVD), wherein the MT-CVD method is preferred for production ofcutting tools as it affords columnar layer structures and by virtue ofthe lower deposition temperature reduces losses of toughness in thesubstrate.

In a further preferred embodiment of the invention the hard materiallayers arranged over the substrate surface and under the α-Al₂O₃ layerand comprising TiN, TiCN and/or TiAlCNO are together of a total layerthickness in the region of 3 μm to 16 μm, preferably in the region of 5μm to 12 μm, particularly preferably in the region of 7 μm to 11 μm.

In a further preferred embodiment of the invention the multi-layercoating starting from the substrate surface has the following layersuccession: TiN—TiCN—TiAlCNO-α-Al₂O₃, wherein optionally a TiN layer, aTiCN layer, a TiC layer or a combination thereof are provided at leastportion-wise over the α-Al₂O₃ layer.

The cutting insert according to the invention can have at leastportion-wise over the α-Al₂O₃ layer one or more further hard materiallayers, preferably a TiN layer, a TiC layer, a TiCN layer or acombination thereof. Such layers are frequently applied as decorativeand/or wear recognition layers to the α-Al₂O₃ layer which appears black,and they themselves are of a yellow-golden or grey-silver colour and canserve as an indicator for use of the tool as those layers are worn awayin metal working. Usually such decorative and/or wear recognition layersare not applied to such surfaces of the tool or after deposition on theentire tool body are removed again from such surfaces, which in metalworking come directly into contact with the metal, for example the rakefaces, as they can have detrimental effects on the working operationdepending on the respective working method and workpiece materialinvolved. Usually the decorative and/or wear recognition layers areabrasively removed from the corresponding surfaces by jet blasting orbrushing treatment. Such abrasive removal of the thin or soft decorativelayers can cause an introduction of compression inherent stresses intothe remaining α-Al₂O₃ layer, but only in the surface-near regions of <1μm depth of penetration so that there is no significant change in theinherent stress condition according to the invention of the α-Al₂O₃layer. By virtue of the slight attenuation of X-ray radiation by α-Al₂O₃that surface-near region is in any case scarcely accessible in terms ofmeasuring technology by X-ray radiographic inherent stress measurementor is so accessible only by extrapolation. With the measurementparameters used here in respect of the sin² ψ method and tilt angles upto ψ=89.5° the ascertained inherent stress of the α-Al₂O₃ layeroriginates from an information depth of about ≧1.5 μm.

For setting the inherent stress condition according to the invention inthe cutting tool, use is made of a jet blasting treatment with a jetblasting agent whose hardness is preferably less than that of theα-Al₂O₃ layer. It is then assumed that the wear mechanism acting on theα-Al₂O₃ layer is essentially shot peening. No substantial removal of theα-Al₂O₃ layer occurs and high compression inherent stresses aregenerated in the substrate body by that mechanism and that method, evenif the total layer thickness of the coating is up to 40 μm in magnitude.

The total layer thickness of the coating is at least 5 μm, preferably atleast 10 μm, particularly preferably at least 15 μm. At excessivelysmall total layer thickness of the coating has the disadvantage thatthere is no longer any guarantee of adequate wear protection by thecoating.

In a further preferred embodiment of the invention the substrate bodycomprises hard metal, preferably containing 4 to 12% by weight of Co, Feand/or Ni, preferably Co, optionally 0.5 to 10% by weight of cubiccarbides of the metals of groups IVb, Vb and VIb of the periodic system,preferably Ti, Nb, Ta or combinations thereof, and WC as the balance.

In a further embodiment of the invention the substrate body compriseshard metal of the above-mentioned composition and has a surface zonewhich is enriched with Co binder phase in relation to the nominaloverall composition of the substrate body and which is depleted of cubiccarbides and which starting from the substrate surface is of a thicknessof 5 μm to 30 μm, preferably 10 μm to 25 μm, wherein the content of Coin the surface zone enriched with Co binder phase is at least 1.5 timeshigher than in the core of the substrate and the content of cubiccarbides in the surface zone enriched with Co binder phase is at most0.5 times the content of cubic carbides in the core of the substrate.

The provision of a surface zone enriched with Co binder phase in thehard metal substrate improves the toughness of the substrate body andopens up a wider area of use of the tool, wherein hard metal substrateswith a surface zone enriched with Co binder phase are preferably usedfor cutting tools for machining steel, whereas cutting tools formachining cast irons are preferably produced without such a surface zoneenriched with Co binder phase.

The present invention also includes a method for production of thecutting insert according to the invention described herein, in whichthere is applied to a substrate body of hard metal, cermet or ceramic bymeans of CVD methods a multi-layer coating which is of a total thicknessof 5 to 40 μm and which starting from the substrate surface has one ormore hard material layers, over the hard material layers an alphaaluminium oxide (α-Al₂O₃) layer of a layer thickness of 1 to 20 μm andoptionally at least portion-wise over the α-Al₂O₃ layer one or morefurther hard material layers as decorative or wear recognition layerswherein the deposition conditions for the α-Al₂O₃ layer are so selectedthat the α-Al₂O₃ layer has a crystallographic preferential orientation,characterised by a texture coefficient TC (0 0 12)≧5 for the (0 0 12)growth direction with

${{{TC}\begin{pmatrix}0 & 0 & 12\end{pmatrix}} = {\frac{I\begin{pmatrix}0 & 0 & 12\end{pmatrix}}{I_{0}\begin{pmatrix}0 & 0 & 12\end{pmatrix}}\left\lbrack {\frac{1}{n}{\sum\limits_{n - 1}^{n}\frac{I({hkl})}{I_{0}({hkl})}}} \right\rbrack}^{- 1}},$wherein

I(hkl) are the intensities of the diffraction reflections measured byX-ray diffraction,

I₀(hkl) are the standard intensities of the diffraction reflections inaccordance with pdf card 42-1468,

n is the number of reflections used for the calculation, and

the following reflections are used for the calculation of TC(0 0 12):

-   -   (0 1 2), (1 0 4), (1 1 0), (1 1 3), (1 1 6), (3 0 0) and (0 0        12),        and after application of the multi-layer coating the substrate        is subjected to a dry or wet jet blasting treatment, preferably        a dry jet blasting treatment, using a granular jet blasting        agent, wherein the jet blasting agent is preferably of a lower        hardness than corundum (α-Al₂O₃) and wherein the jet blasting        pressure, the jet blasting duration and the jet blasting angle        of the jet blasting treatment are so selected that after the jet        blasting treatment the α-Al₂O₃ layer has an inherent stress in        the region of 0 to +300 MPas, and the substrate after the jet        blasting treatment within a region of 0 to 10 μm from the        substrate surface has an inherent stress minimum in the region        of −2000 to −400 MPas.        Measuring Methods

Non-destructive and phase-selective analysis of inherent stresses isonly possible by X-ray diffraction methods (see for example V Hauk.Structural and Residual Stress Analysis by Nondesctructive Methods.Elsevier, Amsterdam, 1997). The sin² ψ method which is widely used (EMacherauch, P Müller, Z. angew. Physik 13 (1961), 305) for X-rayanalysis of inherent stresses is based on the assumption of a homogenousstress condition within the depth of penetration of the X-ray beam andprovides only a mean value for the stress component in one plane.Therefore the sin² ψ method is not suitable for the investigation ofmulti-layer, jet-treated CVD systems in which steep or stepwise changesin the inherent stress are expected within short distances. Instead insuch cases more developed methods such as for example the “UniversalPlot Method” are used, which even in thin layers allow the detection ofinherent stress gradients (Ch. Genzel in: E J Mittemeijer, P Scardi(editor) Diffraction Analysis of the Microstructure of Materials.Springer Series in Material Science, Volume 68 (2004), page 473; Ch.Genzel, Mat. Science and Technol. 21 (2005), 10). As the inherentstresses according to the invention are advantageously achieved by a dryjet blasting treatment using a jet blasting agent whose hardness is lessthan that of corundum (α-Al₂O₃), no or only very slight degrees ofdislocation and only a slight change in the inherent stress are causedin the α-Al₂O₃ layer. Decorative or wear recognition layers optionallyarranged over the α-Al₂O₃ layer are removed by abrasively acting methodswhich for their part change the inherent stress condition in theremaining α-Al₂O₃ layer only in the near surface regions of the layer toabout 1 μm depth of penetration. In the case of the measurementparameters used here the measuring signal in respect of the α-Al₂O₃layer originates from an information depth of about ≧1.5 m. As themeasurement data did not provide any indication of severe inherentstress depth gradients in the α-Al₂O₃ layer they were evaluated usingthe sin² ψ method.

The inherent stresses in the layers were measured in theangle-dispersive diffraction mode on a GE Inspection Technologies(formerly Seifert), 5-Circle-Diffractometer ETA (Ch. Genzel, Adv. X-RayAnalysis, 44 (2001) 247). The parameters used for the measurements andfor determining the inherent stresses are summarised in Table 1hereinafter.

Non-destructive analysis of the inherent stress distribution in theregion of the interface between the substrate body and the coating ispossible only by high-energy X-ray diffraction using intensive parallelsynchrotron radiation. To ascertain the influence of the jet blastingmethod on the condition of the inherent stress in the proximity of thesubstrate surface energy-dispersive diffraction was employed. In thatcase the “modified multi-wavelength method” (as is described in C Stock,Promotionsarbeit, T U Berlin, 2003; Ch. Genzel, C Strock, W Reimers,Mat. Sci. Eng., A 372 (2004), 28), which determines the depth profile ofthe inherent stresses in the substrate to a depth of penetrationdependent on the substrate material. In the case of WC—Co substratesthat depth of penetration is about 10 μm. The experiments were performedon the EDDI (Energy Dispersive Diffraction) material researchmeasurement station which is operated by the Berlin Helmholtz-Zentrumfür Materialien and Energie GmbH on the BESSY Synchrotron Storage Ring(Ch. Genzel, I A Denks, M Klaus, Mat. Sci. Forum 524-525 (2006), 193).The corresponding experimental parameters are set forth in Table 2.

TABLE 1 Experimental parameters for inherent stress analysis of thecoating Radiation CuKα (without Kβ-Filter) 40 kV/45 mA (long fine focus)Diffraction mode Angle-dispersive Optical elements Primary beam:polycapillary half-lens Diffracted beam: parallel beam optics (0.4°Soller aperture + 001-LiF monochromator) Reflections Al₂O₃: 116 (2θ =57.5°) Measurement range in 2θ: 56.0° ≦ 2θ ≦ 59.0°; Δ2θ = 0.05° ψ-range0° . . . 89.5° (sin²ψ = 0 . . . 0.99996) Step width for 0 ≦ ψ ≦ 80°:Δsin²ψ = 0.05°; for ψ > 80°: Δψ = 0.5° Measurement duration 15 s/step in2θ (0.05°) Diffraction line evaluation Pearson VII-function for the Kα₁-and Kα₂- lines Linear absorption coefficient μ_(Al2O3) = 124 cm⁻¹Elastic diffraction constant Al₂O₃: s₁ (116) = −0.474 × 10⁻⁶ MPa⁻¹(DEC)*⁾ ½ s₂ (024) = 2.83 × 10⁻⁶ MPa⁻¹ *⁾Calculated on the basis of themonocrystal elasticity constants of Al₂O₃ (Landoldt-Börnstein, NewSeries, Group III, Volume 11, Springer, Berlin, 1979) and TiN (W. Kress,P. Roedhammer, H. Bilz, W. Teuchert, A. N. Christensen. Phys. Rev. B17(1978), 111.) according to the Eshelby-Kröner model (J. D. Eshelby.Proc. Roy. Soc. (London) A241 (1957), 376; E. Kröner, Z. Physik 151(1958), 504.)

TABLE 2 Experimental parameters for inherent stress analysis in thesubstrate bodies Radiation white synchrotron radiation, E = [10 keV . .. 120 keV] Diffraction mode Energy-dispersive Beam cross-section 0.25 ×0.25 mm² Absorber 2 cm graphite Optics in the diffracted Double gapsystem with an aperture of beam 0.03 × 5 mm² Diffraction angle 2θ = 11°Detector Solid state-LEGe-detector (Canberra) Measurement modesymmetrical ψ-mode (reflection), ψ = 0° . . . 89°, Δψ = 4° for 0 ≦ ψ ≦70° Δψ = 2° for 70° < ψ ≦ 80° Δψ = 1° for 80° < ψ ≦ 89° Measurementduration 60 s/diffractions spectrum Evaluated diffraction lines 001,101, 110, 002, 201, 112 Elastic diffraction constants Taken from B.Eigenmann, E. Macherauch, Mat.-Wiss. u. Werkstofftechn. 27 (1996), 426Calibration With stress-free W powder under the same experimentalconditions

Texture measurements were carried out on a XRD3003PTS diffractometerfrom GE Sensing and Inspection Technologies using Cu K_(α) radiation.The X-ray tube was operated at 40 kV and 40 mA in the spot focus mode.At the primary side a polycapillary half-lens with a measuring apertureof fixed size was used, wherein the blasted surface of the sample was soselected that the X-ray beam is incident only on the coated surface. Atthe secondary side a Soller gap with 0.4° divergence and a 0.25 mm thickNi K_(β) filter were used. Scans were performed in a θ-2θ arrangement inthe angle range of 20°<2θ<100° with a step width of 0.25°. Themeasurements were carried out on a flat surface of the coated cuttinginsert, preferably at the relief face. The measurements were performeddirectly at the aluminium oxide layer as the outermost layer. In thesituation where there is a further layer over the aluminium oxide to bemeasured then that is removed prior to the measurement by a method whichdoes not substantially influence the measurement results, for example byetching. To calculate the texture coefficient TC(0 0 12) the peak heightintensities were used. Background stripping and a parabolic peak fit at5 measurement points were applied to the measured raw data. No furthercorrections of the peak intensities like for example K_(α2) stripping orthin-film layer corrections were implemented.

The jet blasting treatment used for setting the inherent stresscondition according to the invention does not create any significantchange in integral line widths and intensities of the diffractionreflections. The effects of abrasively acting post-treatment methodswhich are used to remove cover layers arranged over the α-Al₂O₃ layerare admittedly slight according to experience, but are not excluded.Therefore measurement of the texture in the case of the cutting insertsaccording to the invention is to be performed at surfaces which are notsubjected to such post-treatment steps, for example at the relief faceof the cutting insert.

The texture coefficient TC(0 0 12) is defined as follows:

${{{TC}\begin{pmatrix}0 & 0 & 12\end{pmatrix}} = {\frac{I\begin{pmatrix}0 & 0 & 12\end{pmatrix}}{I_{0}\begin{pmatrix}0 & 0 & 12\end{pmatrix}}\left\lbrack {\frac{1}{n}{\sum\limits_{n - 1}^{n}\frac{I({hkl})}{I_{0}({hkl})}}} \right\rbrack}^{- 1}},$wherein

I(hkl) are the intensities of the diffraction reflections which asdescribed above are measured by X-ray diffraction and corrected,

I₀(hkl) are the standard intensities of the diffraction reflections inaccordance with pdf card 42-1468,

n is the number of reflections used for the calculation, and

the following reflections are used for the calculation of TC(0 0 12):

-   -   (0 1 2), (1 0 4), (1 1 0), (1 1 3), (1 1 6), (3 0 0) and (0 0        12).

The relative intensity of the (0 0 12) diffraction reflection given bythe texture coefficient TC(0 0 12) is a measurement in respect of the (00 1) preferential orientation or fibre texture of the α-Al₂O₃ layer.Alternatively to evaluation of the (0 0 12) reflection it is alsopossible to evaluate the texture by way of the (0 0 6) diffractionreflection as TC(0 0 6). The use of the (0 0 12) reflection is howeverto be preferred for the coatings according to the invention because the(0 0 6) reflection of the α-Al₂O₃ cannot always be reliably separatedfrom the frequently highly intensive (2 0 0) reflection of TiCN.

EXAMPLES Example 1

WC/Co hard metal substrate bodies (indexable cutting bits of variouscompositions (HM1, HM2, HM3, HM4, HM5 and HM6) were coated in a CVDmethod in the layer sequence TiN-MT-TiCN-α-Al₂O₃—HT-TiCN with variouslayer thicknesses for the individual layers. A thin (<1 μm) binding andnucleation layer of TiAlCNO was deposited between the MT-TiCN layer andthe α-Al₂O₃ layer. All coatings were produced in a CVD reactor of BernexBPX325S type with a radial gas flow.

The MT-TiCN layer was deposited at a pressure of 90 mbars and with thefollowing gas concentrations (percentages in relation to the gases inthe CVD method denote % by volume): 2.0% TiCl₄, 0.5% CH₃CN, 10% N₂,87.5% H₂.

A thin (<1 μm) binding and nucleation layer was deposited between theMT-TiCN layer and the α-Al₂O₃ layer in three process steps.

1. Ti(C,N)—duration: 20 min, temperature: 1000° C., pressure: 500 mbars,gas concentrations: 5% CH₄, 2% TiCl₄, 25% N₂, balance H₂

2. (Ti,Al)(C,N,O)—duration: 15 min, temperature: 1000° C., pressure: 75mbars, gas concentrations: 5% CO, 1% AlCl₃, 2% TiCl₄, 25% N₂, balance H₂

3. (Ti,Al)(C,N,O)—duration: 5 min, temperature: 1000° C., pressure: 175mbars, gas concentrations: 5% CO, 2.5% CO₂, 1% AlCl₃, 2% TiCl₄, 20% N₂,balance H₂

Then the α-Al₂O₃ layer was nucleated by the following method:

1. Flushing with Ar, duration 5 min

2. Treatment with 2% TiCl₄, 2% AlCl₃, balance H₂, at T=1000° C., p=175mbar, duration 5 min

3. Flushing with Ar, duration 5 min

4. Oxidation with 2.5% CO₂, 12% CO, balance H₂ at T=1000° C., p=175mbar, duration 5 min

5. Flushing with Ar, duration 5 min

6. Treatment with 2.5% AlCl₃, balance H₂, at T=1000° C., p=175 mbar,duration 1 min.

For further nucleation a thin α-Al₂O₃ start layer was deposited withoutusing catalytic compounds under the following conditions:

T=1010° C.; p=75 mbars; 2.5% CO₂; 2.0% HCl; 2.0% CO; 2.0% AlCl₃; balanceH₂, duration 40 min.

The growth conditions of the α-Al₂O₃ layer according to the inventionwere selected as follows:

T=1010° C., p=85 mbars, gas concentrations: 91% H₂, 3.0% CO₂, 0.5% H₂S,3.5% HCl, 2.5% AlCl₃. All gas components are introduced simultaneouslyin the specified levels of concentration.

The α-Al₂O₃ layers produced had a very high (0 0 1) preferentialorientation with a texture coefficient TC(0 0 12)>5.

As a reference, hard metal substrate bodies of the same compositionswere also coated with the layer sequence TiN-MT-TiCN-α-Al₂O₃—HT-TiCNwith the same layer thicknesses of the individual layers, wherein a thin(<1 μm) binding and nucleation layer of TiAlCNO was also depositedbetween the MT-TiCN layer and the α-Al₂O₃ layer. Thereupon the α-Al₂O₃layer was nucleated, in accordance with the state of the art.

The growth conditions of the α-Al₂O₃ layer in accordance with the stateof the art were selected as follows:

T=1015° C., p=65 mbars, gas concentrations: 92.3% H₂, 3.5% CO₂, 0.2%H₂S, 2.0% HCl, 2.0% AlCl₃.

The α-Al₂O₃ layers in accordance with the state of the art have only amoderate (0 0 1) preferential orientation.

The compositions of the hard metal substrate bodies used are set forthin Table 3. The layer thicknesses of the individual layers and thetexture coefficient TC(0 0 12) intended for the α-Al₂O₃ layer are setforth in Table 4.

TABLE 3 Hard metal compositions Composition (% by weight) Cubic metalHardness Hard metal Co WC carbide Other HV3 HM1 7 86.5 5 0.5 1500 HM27.5 87 5 0.5 1500 HM3 6 94 — — 1600 HM4 5 86.5 8 0.5 1500 HM5 5.5 86 80.5 1550 HM6 10 81 8.5 0.5 1300

TABLE 4 Layer thickness [μm] MT- HT- Layer system Total TiN TiCN α-Al₂O₃TiCN TC(0 0 12) State of a 17 0.5 9 6 0.5 4.4 ± 0.7 the art State of b20 0.5 9 9 0.5 4.5 ± 0.9 the art State of c 20 0.5 6 12 0.5 4.9 ± 0.4the art State of d 10 0.5 4 4 0.5 2.5 ± 0.4 the art Invention A 17 0.5 96 0.5 6.0 ± 0.2 Invention B 20 0.5 9 9 0.5 6.1 ± 0.3 Invention C 20 0.56 12 0.5 6.2 ± 0.2 Invention D 10 0.5 4 4 0.5 5.5 ± 0.5

The texture coefficients TC[0 0 12] are specified as mean values ofmeasurements on ≧6 various cutting inserts from at least two differentcoating batches.

The cutting bits are then subjected to a jet blasting treatment and theinherent stresses of the α-Al₂O₃ layer and the substrate body aremeasured in the near interface substrate zone (NISZ). The results areset out in Table 5. The value “Inherent stress NISZ substrate” is ineach case the minimum value within the measured inherent stressvariations in the “near interface substrate zone”.

TABLE 5 Results of the inherent stress measurements on cutting insertsInherent stress Inherent stress Inherent stress Hard Layer Jet blastingα-Al₂O₃ TiCN NISZ substrate Cutting insert metal system treatment [MPa][MPa] [MPa]  1 HM1 A unblasted 136 598 −350 (State of the art)  2 HM1 Adry/ZrO₂/ 96 255 −600 (Invention) 5 bar/120 sec  3 HM3 a unblasted 331221 −230 (State of the art)  4 HM3 a dry/ZrO₂/ 197 323 −625 (State ofthe art) 5 bar/120 sec  5 HM3 A unblasted 278 713 −375 (State of theart)  6 HM3 A dry/ZrO₂/ 206 478 −510 (Invention) 5 bar/120 sec  7 HM5 aunblasted 256 330 −290 (State of the art)  8 HM5 a dry/ZrO₂/ 156 325−940 (State of the art) 5 bar/120 sec  9 HM5 A unblasted 308 427 −225(State of the art) 10 HM5 A dry/ZrO₂/ 135 396 −860 (Invention) 5 bar/120sec 11 HM6 a unblasted 369 612 −380 (State of the art) 12 HM6 adry/ZrO₂/ 156 525 −976 (State of the art) 5 bar/120 sec 13 HM6 Aunblasted 411 544 −204 (State of the art) 14 HM6 A dry/ZrO₂/ 147 257−1005 (Invention) 5 bar/120 sec 15 HM2 b unblasted 240 984 −180 (Stateof the art) 16 HM2 b dry/ZrO₂/ 98 595 −580 (State of the art) 5 bar/120sec 17 HM2 B unblasted 224 1280 −55 (State of the art) 18 HM2 Bdry/ZrO₂/ 22 654 −620 (Invention) 5 bar/120 sec 19 HM4 C unblasted 1911140 −280 (State of the art) 20 HM4 C dry/ZrO₂/ 77 733 −720 (Invention)5 bar/120 sec 21 HM2 D unblasted 435 870 −180 (State of the art) 22 HM2D dry/ZrO₂/ 121 −16 −920 (Invention) 5 bar/120 sec

Example 2 Cutting Machining Tests

The cutting bits produced in accordance with Example 1 were used tosubject camshafts to external machining in the interrupted cutting modein accordance with the following test parameters:

-   Work piece: camshaft-   Material: 16MnCr5 (R_(m)=600−700N/mm²)-   Machining; Lengthwise turning in the interrupted cutting mode; wet    machining-   Cutting data: v_(c)=220 m/min    -   f=0.4 mm    -   a_(p)=2.5 mm-   Tool geometry: DNMG150608-NM4-   Tool life: State of the art: cutting insert 11 according to Table 5:    54 components    -   Invention: cutting insert 14 according to Table 5: 80 components

FIG. 1 shows the tool according to the state of the art (cutting insert11 of Table 5) after the machining of 54 components. After the end ofthe tool life both crater wear and also notch wear and break-outphenomena at the cutting edge are to be seen. Crater wear is a typicalform of wear when turning steel materials, occurring because of a lackof wear resistance at high cutting temperature due to thermaloverloading of the tool. The notches and break-outs at the cutting edgein contrast are signs of inadequate toughness of the tool under theselected working conditions.

FIG. 2 shows the cutting insert according to invention (cutting insert14 of Table 5) after the machining of 80 components. The cutting inserthas markedly less pronounced crater wear and no notches.

Example 3 Cutting Machining Tests

Cutting bits produced according to Example 1 were subjected to theso-called strip turning test (test using a severely interrupted cuttingmode). In that test the toughness characteristic of indexable cuttingbits is investigated, by a shaft equipped with four strips ofheat-treatable steel being machined in an external lengthwise turningprocess. The strips which in that case are subjected to cuttingmachining represent only a part of the periphery so that a severelyimpacting action takes place on the tool cutting edges. The life of thetool is determined as the number of entries into the workpieces untilfailure of the cutting edge due to fracture (impact count).

-   Material: 42CrMo4; R_(m)=800 N/mm²-   Machining: Lengthwise turning in the interrupted cutting mode; dry    machining-   Cutting data: v_(c)=170    -   f=0.32 mm    -   a_(p)=2.5 mm-   Tool geometry: CNMG120412-NM4-   Tool life/impact count (respective mean value from 6 tested cutting    inserts):    -   State of the art: Cutting insert 15 according to Table 5: 497        impacts    -   Invention: Cutting insert 18 according to Table 5: 2946 impacts

Example 4 Cutting Machining Tests

Cutting bits produced according to Example 1 were used to subject pumphousings of spheroidal graphite cast iron GGG50 to turning machining(roughing in the interrupted cutting mode) in accordance with thefollowing test parameters:

-   Workpiece: pump housing-   Material: GGG50-   Machining: Turning in the interrupted cutting mode; dry machining-   Cutting data: v_(c)=190 m/min    -   f=0.5 mm    -   a_(p)=3.0 mm-   Tool geometry: WNMA080412-   Tool life: State of the art: Cutting insert 3 according to Table 5:    70 components    -   Invention: Cutting insert 6 according to Table 5: 200 components

The invention claimed is:
 1. A cutting insert: a hard metal, cermet or ceramic substrate body; and a multi-layer coating which is applied thereto by means of CVD methods of a total thickness of 5 to 40 μm and which starting from a surface of the substrate body has one or more hard material layers, over the hard material layers an alpha aluminium oxide (α-Al₂O₃) layer of a layer thickness of 1 to 20 μm and optionally at least portion-wise over the α-Al₂O₃ layer one or more further hard material layers as decorative or wear recognition layers, wherein the α-Al₂O₃ layer has a crystallographic preferential orientation, characterised by a texture coefficient TC (0 0 12)≧5 for the (0 0 12) growth direction with ${{{TC}\begin{pmatrix} 0 & 0 & 12 \end{pmatrix}} = {\frac{I\begin{pmatrix} 0 & 0 & 12 \end{pmatrix}}{I_{0}\begin{pmatrix} 0 & 0 & 12 \end{pmatrix}}\left\lbrack {\frac{1}{n}{\sum\limits_{n - 1}^{n}\frac{I({hkl})}{I_{0}({hkl})}}} \right\rbrack}^{- 1}},$ wherein I(hkl) are the intensities of the diffraction reflections measured by X-ray diffraction, I₀(hkl) are the standard intensities of the diffraction reflections in accordance with pdf card 42-1468, n is the number of reflections used for the calculation, and the following reflections are used for the calculation of TC(0 0 12): (0 1 2), (1 0 4), (1 1 0), (1 1 3), (1 1 6), (3 0 0) and (0 0 12), wherein the α-Al₂O₃ layer has an inherent stress in the region of 0 to +300 MPas, and wherein the substrate within a region of 0 to 10 μm from the surface of the substrate body has an inherent stress minimum in the region of −2000 to −400 MPas.
 2. A cutting insert according to claim 1 wherein the production of the cutting insert includes the substrate body being subjected to a dry or wet jet blasting treatment using a granular jet blasting agent after application of the multi-layer coating.
 3. A cutting insert according to claim 1 wherein the hard material layers arranged over the surface of the substrate body and under the α-Al₂O₃ layer and the hard material layers arranged at least portion-wise optionally over the α-Al₂O₃ layer comprise carbides, nitrides, oxides, carbonitrides, oxynitrides, oxycarbides, oxycarbonitrides, borides, boronitrides, borocarbides, borocarbonitrides, borooxynitrides, borooxocarbides or borooxocarbonitrides of the elements of groups IVa to Vila of the periodic system and/or aluminium and/or mixed metal phases and/or phase mixtures of the afore-mentioned compounds.
 4. A cutting insert according to claim 1 wherein the hard material layers arranged over the surface of the substrate body and under the α-Al₂O₃ layer comprise TiN, TiCN and/or TiAlCNO, wherein the hard material layers respectively involve layer thicknesses in the region of 0.1 μm to 15 μm.
 5. A cutting insert according to claim 1 wherein the hard material layers arranged over the substrate surface and under the α-Al₂O₃ layer and comprising TiN, TiCN and/or TiAlCNO are together of a total layer thickness in the region of 3 μm to 16 μm.
 6. A cutting insert according to claim 1 wherein the multi-layer coating starting from the surface of the substrate body has the following layer succession: TiN—TiCN—TiAlCNO-α-Al₂O₃, wherein optionally a TiN layer, a TiCN layer, a TiC layer or a combination thereof are provided at least portion-wise over the α-Al₂O₃ layer.
 7. A cutting insert according to claim 1 wherein, within a region from the outermost surface region to a depth of 10 μm from the outermost surface of the substrate body, the substrate body has an inherent stress minimum of at least −400 MPas, preferably at least −600 MPas.
 8. A cutting insert according to claim 1 wherein the substrate body comprises hard metal, containing 4 to 12% by weight of Co, Fe and/or Ni, optionally 0.5 to 10% by weight of cubic carbides of the metals of groups IVb, Vb and VIb of the periodic system, or combinations thereof, and WC as the balance.
 9. A cutting insert according to claim 1 wherein the substrate body comprises hard metal and has a surface zone which is enriched with Co binder phase in relation to a nominal overall composition of the substrate body and which is depleted of cubic carbides and which starting from the surface of the substrate body is of a thickness of 5 μm to 30 μm, wherein a content of Co in the surface zone enriched with Co binder phase is at least 1.5 times higher than in a core of the substrate and a content of cubic carbides in the surface zone enriched with Co binder phase is at most 0.5 times a content of cubic carbides in the core of the substrate.
 10. A method for production of a cutting insert according to claim 1, comprising: applying to a substrate body of hard metal, cermet or ceramic by means of CVD methods a multi-layer coating which is of a total thickness of 5 to 40 μm and which starting from a surface of the substrate body includes one or more hard material layers, over the hard material layers an alpha aluminium oxide (α-Al₂O₃) layer of a layer thickness of 1 to 20 μm and optionally at least portion-wise over the α-Al₂O₃ layer one or more further hard material layers as decorative or wear recognition layers, wherein deposition conditions for the α-Al₂O₃ layer are so selected that the α-Al₂O₃ layer has a crystallographic preferential orientation characterised by a texture coefficient TC (0 0 12)≧5 for the (0 0 12) growth direction with ${{{TC}\begin{pmatrix} 0 & 0 & 12 \end{pmatrix}} = {\frac{I\begin{pmatrix} 0 & 0 & 12 \end{pmatrix}}{I_{0}\begin{pmatrix} 0 & 0 & 12 \end{pmatrix}}\left\lbrack {\frac{1}{n}{\sum\limits_{n - 1}^{n}\frac{I({hkl})}{I_{0}({hkl})}}} \right\rbrack}^{- 1}},$ wherein I(hkl) are the intensities of the diffraction reflections measured by X-ray diffraction, I₀(hkl) are the standard intensities of the diffraction reflections in accordance with pdf card 42-1468, n is the number of reflections used for the calculation, and the following reflections are used for the calculation of TC(0 0 12): (0 1 2), (1 0 4), (1 1 0), (1 1 3), (1 1 6), (3 0 0) and (0 0 12), wherein after application of the multi-layer coating the substrate body is subjected to a dry or wet jet blasting treatment using a granular jet blasting agent, and wherein the jet blasting pressure, the jet blasting duration and the jet blasting angle of the jet blasting treatment are so selected that after the jet blasting treatment the α-Al₂O₃ layer has an inherent stress in the region of 0 to +300 MPas, and the substrate after the jet blasting treatment within a region of 0 to 10 μm from the surface of the substrate body has an inherent stress minimum in the region of −2000 to −400 MPas.
 11. A method for production of a cutting insert according to claim 10 wherein the blasting treatment is a dry jet blasting treatment.
 12. A method for production of a cutting insert according to claim 10 wherein the jet blasting agent has a lower level of hardness than corundum (α-Al₂O₃).
 13. A cutting insert according to claim 2 wherein the blasting treatment is a dry jet blasting treatment.
 14. A cutting insert according to claim 2 wherein the jet blasting agent has a lower level of hardness than corundum (α-Al₂O₃).
 15. A cutting insert according to claim 4 wherein the hard material layer is TiAlCNO arranged directly under the α-Al₂O₃ layer and has a layer thickness in the region of 0.1 μm to 1 μm.
 16. A cutting insert according to claim 4 wherein the hard material layer is TiN or TiCN with a layer thickness in the region of 2 μm to 15 μm.
 17. A cutting insert according to claim 16 wherein the layer thickness of the hard material layer of TiN or TiCN is in the region of 3 μm to 10 μm.
 18. A cutting insert according to claim 5 wherein the hard material layers arranged over the substrate surface and under the α-Al₂O₃ layer and comprising TiN, TiCN and/or TiAlCNO have a total layer thickness in the region of 5 μm to 12 μm.
 19. A cutting insert according to claim 18 wherein the hard material layers arranged over the substrate surface and under the α-Al₂O₃ layer and comprising TiN, TiCN and/or TiAlCNO have a total layer thickness in the region of 7 μm to 11 μm.
 20. A cutting insert according to claim 7 wherein the inherent stress minimum is at least −600 MPas.
 21. A cutting insert according to claim 20 wherein the inherent stress minimum is at least −800 MPas.
 22. A cutting insert according to claim 8 wherein the substrate body comprises hard metal containing 4 to 12% by weight of Co.
 23. A cutting insert according to claim 8 wherein the substrate body comprises 0.5 to 10% by weight of cubic carbides of Ti, Nb, Ta or combinations thereof. 